A mobile device can be localized by a multilateration method in which the mobile device measures the time difference between some specific reference signals originating from several base stations deployed in the network and reports these time differences to a specific device in the network. The specific device calculates the position of the mobile device based on these time differences and knowledge of the locations of the base stations. The mobile device must measure time differences from at least three base stations to support a two-dimensional position determination. The two-dimensional position determination does not comprise a determination of the altitude of the mobile device. The accuracy of the multilateration method depends on the resolution of the time difference measurements, the geometry of the neighboring base stations and the signal environment.
In mobile communications a plurality of localization methods based on multilateration have been standardized so far, for example Enhanced Observed Time Difference (E-OTD), Observed Time Difference of Arrival (OTDOA) and Wireless Assisted Global Positioning System (A-GPS).
E-OTD has been designed for GSM (Global System of Mobile Communications) and GPRS (General Packet Radio Service). In GSM, the mobile device monitors transmission bursts from a plurality of neighboring base stations and measures the time differences between the arrivals of the GSM frames from the base stations to which it is communicating. These observed time differences are used to trilaterate the position of the mobile device.
OTDOA has been designed for UMTS (Universal Mobile Telecommunications System) and LTE (Long Term Evolution). The OTDOA location server estimates the position of a mobile device by referencing the timing of signals as they are received at the mobile device from a minimum of three base stations. The position of the mobile device is at the intersection of at least two hyperbolas defined by the observed time differences of arrival of the UMTS or LTE frames from multiple base stations.
Specifically in LTE, the UE (user equipment=mobile device) is required to detect positioning-specific reference signals (PRS) and/or alternatively the cell-specific reference signals (CRS) transmitted from several eNBs (evolved NodeB=base station) and measure the reference signal time difference (RSTD) between a chosen positioning reference cell eNB and each other detectable positioning neighbor cell eNB using the PRS. The network is not obligated to be synchronized at the transmit antenna connectors. The UE reports the estimated RSTD values to the network in the specified LTE positioning protocol (LPP), in particular to the evolved service mobility location centre (E-SMLC). For the RSTD measurement, the UE is supported by the E-SMLC, which provides assistance data to the UE via LPP, e.g. list of positioning cell IDs, CPs, PRS occasions, PRS offsets and repetition periods, search window, etc. The UE has to estimate the RSTD values with a minimum accuracy requirement. The serving cell, to which the UE is perfectly synchronized, must be in the set of positioning cell IDs to recover the system frame number (SFN) of at least one positioning cell eNB.
The search window to be used can be derived from assistance data conveyed by the E-SMLC to the UE and limits the search range, in which the UE has to search for PRS occasions. The maximum time offset between the centre of the search window and the reference cell PRS occasion is limited to 800 us, cell maximum span of the search window is limited to 199.8 us. The latter time value sets the actual search range for any applied RSTD estimation algorithm.
The search window of 199.8 us equals 2.8 OFDM symbol frames (including CP) or 0.2 subframes. The PRS are detected after FFT processing. The UE is only synchronized to the serving cell. In the beginning of the RSTD estimation, the UE is not synchronized to any other cell, e.g. none of the positioning cells. Any PRS-/CRS-based estimation method, which accurately estimates the exact OFDM (orthogonal division multiplexing) frame time offset between the positioning reference cell and the positioning neighbor cell has a limited capture range. The synchronization algorithm resolving this time sample offset is referred as fine timing. The capture range of fine timing is defined by the frequency distance between two subsequent PRS/CRS resource elements (REs), e.g. their subcarrier distance. Beyond this time range, the estimation becomes ambiguous due to the cyclic property of the FFT (Fast Fourier Transform) exponentials. For an OFDM system the capture range depends on the subcarrier spacing. For an LTE system with 15 kHz subcarriers spacing, the maximum capture range results in 66.66 us= 1/15 kHz, which is much smaller than the maximum search window span of 199.8 us. Therefore an additional OFDM frame synchronization is necessary to cover the maximum search window which can be achieved by a sliding window approach.
A sliding window approach, however, is time consuming. This problem arises with any mobile communication technology or standard in which the capture range of the mobile device is smaller than the size of the search window. Thus, a solution that speeds up the reference signal time difference estimation is desirable.